Analysis of cells by patch clamping is a powerful electrophysiological recording technique. Patch clamping is used to study electrical properties of cell membranes, particularly activity and regulation of ion channels included in the membranes. This technique has gained popularity as a measurement tool because it is one of the most direct and meaningful ways to study how the activity of ion channel proteins is modulated by physiological factors in general, and pharmaceutical compounds in particular.
Patch clamping was developed as a procedure performed manually. In traditional patch clamping, a glass pipet of small diameter is placed against the membrane of a cell. Application of a vacuum to fluid in the interior of the pipet pulls the membrane against the end of the pipet, creating a tight, resistive seal between the perimeter of the pipet end and a “patch” on the membrane. This seal, often termed a gigaseal because of its gigohm or near-gigohm resistance, directs electrical current along a path from the bore of the pipet through the patch and/or the whole cell. When the patch is permeabilized selectively, the electrical properties of the remainder of the cell membrane, other than the patch, may be measured in a whole-cell analysis. Alternatively, the remainder the cell membrane may be removed, leaving only the patch to be analyzed.
In either case, a current or voltage applied across the cell or patch membrane may be measured. For example, a “stimulation” voltage may be applied between 1) an “external” electrolytic fluid that holds the cell (or patch) and 2) an “internal” electrolytic fluid in the interior of the pipet. Such a stimulation voltage produces a corresponding response in ion flow (and thus the electrical current) through the whole cell membrane or membrane patch. The impedance of the membrane determines the magnitude of the current. Ligand- and/or voltage-induced changes in ion channel activity thus produce corresponding changes in the impedance of the membrane and in the magnitude of the current. In a typical approach, the voltage (or current) may be fixed or “clamped” during analysis, while the resultant current (or voltage) is measured, thereby producing a voltage-(or current) clamped analysis.
Despite the sensitivity of the patch-clamp method, the manual approach with a pipet may not be suitable for analysis of libraries of compounds, such as in high-throughput drug screens. In particular, with the manual approach, cells are analyzed one at a time. Accordingly, testing the effects of chemical compounds may be too slow to screen large numbers of candidate compounds.
Efforts to improve the speed of patch-clamp analysis have focused on analyzing more than one cell at once. For example, the single pipet may be replaced with an array of apertures defined by a planar material. With such a “planar patch-clamp” device, cells may be disposed at each of the apertures and electrically monitored with circuits specific for each aperture.
Despite their potential for increased throughput, these planar patch-clamp devices may be inadequate for a number of reasons. For example, some patch-clamp devices use movable electrodes that are not dedicated to individual apertures. Such devices may limit the number of apertures that can be excited and monitored in parallel and increase the incidence of mechanical malfunction with moving parts. Other patch-clamp devices may include dedicated monitoring circuits but lack integration of their circuitry, for example, having a separate sensor circuit for each aperture. Such separate circuits place a practical limit on the density of apertures that may be included, based on the size and complexity of the resulting electrical interface. Accordingly, this aperture limit places a corresponding limit on the number of cells/compounds/conditions that may be analyzed in an experiment. Furthermore, such insufficiently integrated devices may not allow the conditions at individual apertures to be monitored and modified automatically and selectively.